1. Field of the Invention
The present invention relates to a wiring substrate on which an electronic component such as a semiconductor element, a capacitive element, a resistor or the like is mounted, and that includes a wiring conductor on its surface, the wiring conductor being coated with a plated metal layer by electroless plating. The invention also relates to a method for producing the same.
2. Description of the Related Art
Conventionally, a wiring substrate on which an electronic component such as a semiconductor element, a capacitive element, a resistor or the like is to be mounted generally includes a substantially rectangular plate-shaped insulator and a plurality of wiring conductors. The insulator is made of an aluminum oxide sintered body and has a mounting portion for mounting an electronic component. The plurality of wiring conductors are made of a metal material having a high melting point such as tungsten, molybdenum or manganese are formed so as to lead from the mounting portion to the outer circumference of the insulator. An electronic component such as a semiconductor element, a capacitive element and a resister is mounted on the mounting portion of the insulator, and each electrode of the electronic component is electrically connected to the wiring conductor via a conductive connecting member such as a solder and a bonding wire.
In such a wiring substrate, a portion of the wiring conductor that is led out to the outside is connected to a circuit wiring of an external electrical circuit board via a solder or the like, so that the wiring substrate is mounted on the external electrical circuit, and each electrode of the electronic component mounted on the wiring substrate is electrically connected to a predetermined external electrical circuit.
Furthermore, in such a wiring substrate, the surface of the wiring conductor is coated with a plated metal layer such as nickel to improve the wettability and the bonding properties of solders and bonding wires with respect to the wiring conductor made of a metal material having a high melting point.
As a method for forming a coating of a plated metal layer such as nickel, electroless plating, which does not require leads, has come to be used more commonly, because higher density of the wiring conductors resulting from compactness of the wiring substrate makes it difficult to form leads for supplying power for plating.
A coating of a plated metal layer such as nickel on the wiring conductors by electroless plating is formed by the following method. Since metals having a high melting point such as tungsten, molybdenum or manganese do not have catalytic activity with respect to reduction precipitation of metals such as nickel by electroless plating (autocatalytic type), in general, first, the surface of the wiring conductor is coated with an element having catalytic activity with respect to precipitation of nickel or the like, such as an element of Group 1B (copper, silver, or gold), to provide the surface thereof with catalytic activity. Thereafter, the wiring conductor is immersed in an electroless plating solution so that the surface is coated with a plated metal layer by the action of the catalytic activity of the element of Group 1B. In general, this method is performed in the following manner.
First, an insulator having wiring conductors on its surface is prepared.
Next, the wiring conductors are immersed in an active liquid obtained by adding additives such as a pH adjuster and a complexing agent to an aqueous solution containing at least a metal compound that serves as a supply source of an element of Group 1B such as copper chloride, and a lead compound such as lead chloride, so that the element of Group 1B such as copper is precipitated on the surface of the wiring conductor for coating. Then, the wiring conductor is immersed in an active liquid obtained by adding additives such as a complexing agent and a pH adjuster to an aqueous solution containing at least a metal compound that serves as a supply source of a plated metal such as nickel sulfide or nickel chloride, and a lead compound such as lead chloride, so that the element of Group 1B such as copper is precipitated on the surface of the wiring conductor for coating.
Then, the wiring conductors are immersed in an electroless plating solution obtained by adding a completing agent, a pH buffer, a stabilizer and the like to an aqueous solution containing at least a nickel compound that serves as a supply source of nickel such as nickel sulfide or nickel chloride, and a reductant such as sodium hypophosphite or dimethylamine borane, so that the nickel in the plating solution is reduced and precipitated by the action of the catalytic activity of the element of Group 1B such as copper that is coating the surfaces of the wiring conductors, so that a nickel plated metal layer is formed selectively only on the surfaces of the wiring conductors.
The lead compound contained in the active liquid is first adsorbed on the surface of the wiring conductor when the wiring conductors made of a metal having a high melting point are immersed in the active liquid, and the lead compound acts so as to make the surfaces of the conductors sensitive to the precipitation and the coating of the element of Group 1B such as copper. Thus, the lead compound facilitates the precipitation and the coating of the element of Group 1B such as copper on the wiring conductors and makes it possible to form a uniform coating. Furthermore, the element of Group 1B such as copper coating the surface of the wiring conductor and the lead remain and are contained in the electroless plated metal layer coating the surfaces of the wiring conductors.
Another method for forming a coating of a plated metal layer such as nickel and copper on the wiring conductor by electroless plating is as follows. Since metals having a high melting point such as tungsten, molybdenum or manganese do not have catalytic activity with respect to reduction and precipitation by electroless plating (autocatalytic type) of metals such as nickel or copper, in general, first, the surface of the wiring conductor is coated with a platinum group metal such as palladium or platinum to provide the surface thereof with catalytic activity. Thereafter, the wiring conductor is immersed in an electroless plating solution so that the surface is coated with a plated metal layer. In general, this method is performed in the following manner.
First, an insulator having wiring conductors on its surface is prepared.
Next, the wiring conductors are immersed in an active liquid obtained by adding additives such as a pH adjuster such as sodium hydroxide or potassium hydroxide to an aqueous solution containing at least a metal compound that serves as a supply source of a platinum group metal such as palladium chloride, and a lead compound such as lead chloride, so that the platinum group metal such as palladium is precipitated on the surface of the wiring conductor for coating.
Then, the wiring conductors are immersed in an active liquid obtained by adding a completing agent, a pH buffer, a stabilizer and the like to an aqueous solution containing at least a metal compound that serves as a supply source of a plated metal such as nickel sulfide or copper sulfide, and a reductant such as sodium hypophosphite, dimethylamine borane, formalin, so that a metal such as nickel or copper is reduced and precipitated by the action of the catalytic activity of the platinum group metal such as palladium coating the surfaces of the wiring conductors, so that a plated metal layer is formed selectively only on the surfaces of the wiring conductors.
The lead compound contained in the active liquid is first adsorbed on the surface of the wiring conductor when the wiring conductors made of a metal having a high melting point is immersed in the active liquid, and thus the lead compound acts so as to make the surfaces of the conductors sensitive to the precipitation and the coating of the platinum group metal such as palladium. Thus, the lead compound facilitates the precipitation and the coating of the platinum group metal such as palladium or platinum on the wiring conductors and makes it possible to form a uniform coating. Furthermore, the platinum group metal such as platinum or palladium coating the surface of the wiring conductor and the lead remain and are contained in the electroless plated metal layer coating the surfaces of the wiring conductors.
The conventional wiring substrate contains lead in the plated metal layer, as described above, so that when heat is applied to the plated metal layer, the lead moves and is diffused onto the surface of the plated metal layer and oxidized there, which causes a functional problem that the color is changed spottedly and environmental and security problems that the lead in the plated metal layer harms the human body.
Furthermore, it can be one solution not to let lead contained in the active liquid in order to solve the problems. However, in this case, since the surface of the wiring conductor made of a metal having a high melting point is not sufficiently sensitive to the precipitation and the coating of elements of Group 1B such as copper or platinum group metals such as palladium or platinum, the elements of Group 1B cannot be precipitated on the surfaces of the wiring conductors uniformly and firmly for coating. As a result, roughness, cracks, swelling or the like may be generated.